Relevance of SEM to Long-Term Mechanical Properties of Cemented Paste Backfill

This paper is an attempt to relate the microstructure to long-term mechanical properties of the cemented paste backfill produced from a hard rock mine tailing from North Queensland in Australia bound with flyash-based geopolymer (geopolymer), flyash-blended cement (FBC), and general purpose cement (GPC). A relatively high slump (260 mm) paste backfill mix with 74 wt% solids has been used to prepare cylindrical paste backfill samples with a diameter of 50 mm and a height of 100 mm. The uniaxial compressive strength tests were conducted on all samples after curing for 112 days to obtain their strength, failure strain and Young’s modulus. Fractured samples were examined under scanning electron microscope to understand the failure mechanisms at the microstructural scale. The results show that binders significantly affected the mechanical properties of paste backfills (ANOVA, p < 0.05). The paste backfill bound with geopolymer gave the lowest strength and Young’s modulus, while the paste backfills bounded with FBC and GPC showed comparable higher strength and modulus values. This was attributed to the relatively well-packed paste backfills with less cracks and smaller pore sizes in these paste backfills bound with FBC and GPC binders. In particular, needle-shaped particles, which were originally identified in GPC, highly influenced the mechanical property of paste backfills. These results indicate that fly ash can be used to partially replace the cement as a binder for paste backfills to achieve economic and environmental benefits.     

Laboratory Investigation of the Effects of Temperature on the Mechanical Properties of Sandstone

An experimental study has been carried out as part of a wider programme of research in order to examine some of the physico-mechanical properties of the Jiaozuo Sandstone after exposure to extremely high temperatures. The mechanical properties of the rock under examination are discussed before the results of the thermo-mechanical response of the sandstone are presented and analysed. The range of temperature to which the sandstone has been exposed is 20–1,200 °C. The physical properties considered include the shape, volume, mass and density changes and the velocity of longitudinal and transverse elastic waves through the samples, before and after exposure to high temperature. The mechanical properties considered include the stress–strain response, the uniaxial compressive strength, the modulus of elasticity and the Poisson’s ratio. The results are analysed and discussed and possible mechanisms for the observed thermo-mechanical response, are postulated.     

Influence of Shear Strain on the Poisson’s Ratio of Clean Sands

In the present study, a series of resonant column tests was performed to determine the influence of confining pressure, shear strain and relative density on the dynamic properties and Poisson's ratio of poorly graded clean sand. The tests were performed on the sand specimens of size 50 × 100 mm compacted at relative densities 30, 50 and 75 %. To achieve the corresponding relative density, sand was compacted in 5 equal layers with a specific number of blows. A fixed-free type resonant column apparatus was used to determine the dynamic soil properties at various confining pressures. It has been inferred from the data that the shear modulus (\(G\)) increases with an increase in confining pressure and relative density; and decreases with an increase in shear strain. In addition, damping ratio (\(D\)) decreases with an increase in confining pressure and relative density; and increases with an increase in shear strain. In addition, Poisson’s ratio (\(\nu\)) decreases with an increase in confining pressure and relative density and increases with an increase in the shear strain. The variation of shear modulus with Poisson’s ratio is also discussed. It has been found that there has been a decrease in shear modulus with an increase in Poisson’s ratio of the soil. It is noticed that the small strain shear modulus determined from the present study closely matches with the value determined using the correlations from the literature.     

岩石力学性质的应变率效应试验

以玄武岩为试样,开展了中低应变率下的岩石单轴抗压试验。本次研究分析应变率对岩石抗压强度、弹性模量、泊松比等力学参数的影响,分别提出了岩石抗压强度、弹性模量、泊松比等参数与应变率之间的拟合关系式。以峰值应力对应的应变和峰值后的软化模量为脆性评价指标,分析加载应变率对岩石脆性的影响。研究表明:(1)岩石抗压强度、弹性模量均随应变率的增加而增加。(2)岩石泊松比随应变率增加而减小。(3)随着应变率增加大,峰值应力对应的应变增大,峰值后的应变软化程度减小,岩石脆性减弱。(4)应变率对岩石抗压强度影响较大,对弹性模量和泊松比的影响较小。(5)曲线拟合效果良好,提出的拟合关系式合理。     

Different bedding loaded coal mechanics properties and acoustic emission

Using TAW-2000KN electro-hydraulic servo rock press machines and the American Physical Acoustics Company’s SH-II acoustic emission systems, experimental studies began to address the mechanical properties in different beddings of loaded coal and the related acoustic emission characteristics, established based on the acoustic emission damage model, and verify the model. The results show that the mechanical properties of different coal sample beddings are distinctive, with maximum uniaxial compressive strength and elastic modulus of vertical stratification of coal samples and the minimum Poisson’s ratio. Thus, the minimum uniaxial compressive strength and elastic modulus of oblique bedding coal samples along with the maximum Poisson’s ratio in the processes of loading result in different bedding coal samples having different stress–strain curves, especially when different bedding coal samples experience the stages of fissure compression, elastic deformation, plastic deformation and instability and destruction. In addition, the displacement proportions of each stage of the loading process have relatively obvious differences: the loading times of vertical, parallel and oblique bedding coal are 495, 382 and 331 s, respectively, and their acoustic emission mutation points of peak stress are approximately 60, 41 and 33%, respectively. Thus, we can use the mutation point as precursor information to estimate the damage intensity in different bedding seams. The theoretical and experimental stress–strain curves obtained by the coal damage model are basically identical, verifying the reliability of the model and reflecting the feasibility of acoustic emission technology in the study of coal damage. The results can effectively forecast coal and gas outburst hazard in coal mines, especially highly gassy and outburst mines. It can also make comprehensive predictions for flooding accidents, roof fall accidents and other disasters, and provide valuable evacuation time for underground coal mine workers. The results are of great scientific significance in safeguarding the safety of coal mines.     

Predicting elastic properties of schistose rocks from unconfined strength using intelligent approach

Elastic properties of rocks play a major and crucial role for the design of any engineering structure. Determination of elastic properties in laboratory is tedious, laborious, very time consuming, as well as expertise is required, whereas determination of uniaxial compressive strength (UCS) and tensile strength in laboratory is simple, easy, and less expertise is required. Here, an attempt has been made to predict the elastic properties (Poisson’s ratio and Young’s modulus) of the schistose rocks from unconfined strength (UCS and tensile strength) using artificial neural network (ANN). A three-layer feed-forward back propagation neural network with 2-5-2 architecture was trained up to 855 epochs to predict the elastic properties of rock mass. The network was trained and tested by 120 data sets, and validation of the network was done by 20 new randomly selected data sets of UCS and tensile strength. The samples were collected from the schistose rocks of Nathpa-Jhakri hydropower project site, SJVNL, Himachal Pradesh, India. To check the validity and suitability of the artificial neural network technique, multivariate regression analysis (MVRA) is also performed, and comparison has been made. It was found that ANN gives closer values of predicted Poisson’s ratio and Young’s modulus as compared to MVRA. The coefficient of determination for Poisson’s ratio was 0.9809 and 0.843 by ANN and MVRA, respectively, whereas 0.9922 and 0.9362 for Young’s modulus by ANN and MVRA, respectively. The mean absolute percentage error (MAPE) for Young’s modulus is 11.13 and 28.21 by ANN and MVRA, respectively; whereas MAPE for Poisson’s ratio is 3.64 and 9.23 by ANN and MVRA, respectively.     

Variation of mechanical properties of granite after high-temperature treatment

Variations in the mechanical properties (compressive strength, elastic modulus, tensile strength, and fracture toughness) of granite were analyzed as functions of temperature. It was found that above 200 °C, tensile strength and fracture toughness tended to decrease with temperature, while variations in the compressive strength and elastic modulus demonstrated decreasing trends when the heating temperature exceeded 400 °C. The temperature ranges of room temperature—200 and above 600 °C—corresponded to an undamaged state and strongly/completely damaged state, respectively. It is suggested that 400 °C might be a critical threshold of thermal damage to granite. Based on results of statistical tests, a sharp decrease in mechanical properties can be recognized, accompanied by a drastic growth in peaking strain and acoustic emission rate. This phenomenon may be associated with the α/β phase transition of quartz.     

Effect of Temperature on the Strength Parameters at the Plastic Domain for Unconsolidated Sandstones

Heavy and extra heavy oil reservoirs are subjected to high temperatures during thermal recovery processes like SAGD and CSS. In-situ temperature state changes throughout these operations may generate variations in the mechanical properties or strength parameters of the rock such as Young and Bulk’s modulus, internal friction angle, cohesion and dilation angle. This paper presents results about the variation of the elastic modulus and mechanical properties, in both elastic and plastic region due to changes in temperature and effective confining stress for reconstituted samples of Colombian unconsolidated sands. In order to study these changes, several drained triaxial tests are performed in a range of temperature between 50 and 230 °C and a range of effective confining stress between 0.4 and 8.2 MPa, these tests are carried out on reconstituted samples of oil sands recovered in an outcrop in the Magdalena middle valley of Colombia. The experimental results are analyzed with an analytical model based on the constitutive model of Mohr–Coulomb. The analysis is divided in two ways: First, the elastic region is analyzed calculating the variation of the elastic modulus (Young and bulk’s modulus) and the mechanical parameters (cohesion and internal friction angle) with temperature and effective confining stress, these calculus are done in the linear zone of the stress–strain curve. Then, the plastic region is analyzed using Mohr–Coulomb constitutive model with a non-associative flow rule in order to represent in a better way the dilatant/contractive behavior of the material, from this analysis is possible to obtain the behavior of the mechanical properties during plastic strains. The stress–strain curves obtained from the triaxial compression tests show that for the highest and medium effective confining stress (8.2 and 4.0 MPa), there is a hardening strain behavior after the yielding point that implies a loose sand behavior under confining conditions. Meanwhile, for lowest effective confining stress (0.4 MPa) the samples show a softening strain, a typical behavior for dense sands under confining conditions.     

渗透压-应力耦合作用下煤体常规三轴试验的颗粒流模拟

为了研究在渗透压-应力耦合作用下围压和瓦斯压力对煤体变形破坏规律的影响,借助PFC2D软件进行了340组抗压及抗拉试验模拟,建立了煤体宏观力学参数与细观参数之间的关系。利用平行黏结模型和推导得出的细观参数,对煤体进行了不同围压和瓦斯压力条件下常规三轴试验的颗粒流模拟,并将模拟结果与物理试验结果进行对比及误差分析,对煤体宏观力学参数的变化规律进行了总结。分析结果表明,经过推导得到的宏细观参数关系,可以控制模拟结果的误差在10%之内,能较好地模拟煤体的力学特征;围压增大时,煤体的抗压强度、残余强度及弹性模量呈增大趋势,泊松比以及煤体的破裂角度变小,与物理试验所得围压对煤体的刚度和强度起增强作用的结论一致;瓦斯压力的升高会导致煤体抗压强度,残余强度及弹性模量减小,泊松比以及煤体破裂角度变大,验证了物理试验中瓦斯压力对煤体强度具有劣化作用的结论。     

Analysis of size effects on the geomechanical parameters of intact granite samples under unconfined conditions

A total of 28 uniaxial compressive strength tests were performed on cylindrical Blanco Mera granite samples with diameters ranging between 14 and 100 mm, with results indicating that this granite undergoes a significant reverse size effect: the UCS increases as sample diameter increases up to 54 mm, but thereafter decreases. It was also found that the results tend to be more scattered for smaller sample diameters. We also found an apparent correlation between Young’s modulus and sample diameter. It was not possible to draw any clear conclusions regarding the variability in Poisson’s ratio with sample size. With respect to crack initiation and crack damage stresses, the behaviour of the tested samples also indicates a reverse effect. This research would suggest that the traditionally assumed decrease in strength as sample size increases does not hold for granite samples with diameters below 54 mm.     

Strain Rate Dependency of Coarse Crystal Marble Under Uniaxial Compression: Strength,Deformation and Strain Energy

Strain rate during testing, uniaxial or triaxial, has important influence on the measured mechanical properties of rocks. Uniaxial compression tests were performed at nine pre-specified static-to-quasistatic strain rates (ranging from 1 × 10^?5 to 1 × 10^?1 s^?1) on coarse crystal marble. The aim is to gain deep insight into the influence of strain rate on characteristic stresses, deformation properties and conversion of strain energy of such rock. It is found that the strain rate of 5 × 10^?3 s^?1 is the threshold to delineate the failure modes the tested coarse marble behaves in. At a strain rate less than this threshold, single-plane shear and conjugate X-shaped shear are the main failure modes, while beyond this threshold, extensile and splitting failures are dominant. The stress for crack initiation, the critical stress for dilation, the peak stress, and Young’s modulus are all found to increase with strain rate, with an exception that the above stresses and modulus appear relatively low compared to the strain rate in the range of between 1 × 10^?4 and 5 × 10^?3 s^?1. The pre-peak absorbed strain energy, damage strain energy and elastic strain energy are found to increase with strain rate. In addition, the elastic strain energy stored before peak point favors brittle failure of the specimen, as the more stored elastic energy in the specimen, the stronger the fragmenting.     

Plane Strain Testing with Passive Restraint

A plane strain condition for testing rock is developed through passive restraint in the form of a thick-walled cylinder. The so-called biaxial frame generates the intermediate principal stress that imposes a triaxial state of stress on a prismatic specimen. Major and minor principal stresses and corresponding strains are accurately measured, providing data to calculate the elastic (Young’s modulus and Poisson’s ratio), inelastic (dilatancy angle), and strength (friction angle and cohesion) parameters of the rock. Results of experiments conducted on Indiana limestone in plane strain compression are compared with the results of axisymmetric compression and extension. With proper system calibration, Young’s modulus and Poisson’s ratio are consistent among the tests. The plane strain apparatus enforces in-plane deformation with the three principal stresses at failure being different, and it allows one to determine the Paul-Mohr-Coulomb failure surface, which includes an intermediate stress effect.     

弹性模量对岩样破坏前兆及全部变形特征的影响

利用FLAC内嵌语言编制的计算平面应变压缩岩样轴向、侧向、体积应变及泊松比的FISH函数,计算了弹性模量不同时单缺陷岩样的全部变形特征,研究了弹性模量对岩样的破坏过程及前兆的影响。在峰前及峰后,岩石的本构模型分别取为线弹性模型及莫尔-库仑剪破坏与拉破坏复合的应变软化模型。当弹性模量不是较高时,岩样自始至终仅出现一条倾斜的剪切带。当弹模较高时,最终剪切应变仅强烈集中于贯通岩样的剪切带内部。剪切带倾角在Arthur与Coulomb倾角之间,且随弹模的增加而降低,经典理论对此不能解释。随着弹模的增加,峰值应力增加,峰前的应力-轴向应变曲线变得陡峭,峰后的应力-轴向应变曲线的斜率几乎不变。随着弹模的增加,峰值强度所对应的轴向应变及侧向应变的大小均降低;应力-侧向应变曲线软化段变得陡峭。随着弹模的降低,侧向应变-轴向应变曲线、泊松比-轴向应变曲线及体积应变-轴向应变曲线在峰前发生转折（偏离线性状态）的程度越来越大;非弹性轴向应变增加;材料缺陷附近的最大剪切应变增量增加。岩样破坏的前兆随着弹性模量的降低而逐渐增强。     

随机分布贯穿裂隙岩体变形特性研究

洞室开挖后,其周边通常会产生许多随机分布的贯穿裂隙,直接影响洞室围岩稳定,研究随机分布贯穿裂隙岩体的变形及变形特性具有重要意义。基于线弹性理论和线性刚度理论计算岩石和裂隙的位移,用概率的方法建立了随机分布贯穿裂隙岩体变形的计算模型,给出了随机分布贯穿裂隙岩体的等效弹性模量和等效泊松比,研究了岩石和裂隙的材料参数和几何参数对岩体等效弹性模量和等效泊松比的影响。可得如下结论：等效弹性模量和等效泊松比随着岩石弹性模量的增大而增大;等效泊松比随着岩石泊松比的增大而增大;等效弹性模量和等效泊松比随着裂隙法向刚度的增大而增大;随着剪切刚度的增大,等效弹性模量逐渐增大,而等效泊松比则逐渐减少;随着裂隙平均间距的增大,等效弹性模量逐渐减小,等效泊松比在平均倾斜角较小时逐渐增大,在平均倾斜角较大时逐渐减小;随着裂隙平均倾斜角的增大,等效弹性模量先减小后增大,而等效泊松比先增大后减小。模型能较全面地考虑构成岩体的岩石和裂隙的材料参数与几何参数对岩体变形的影响,其结果对研究洞室围岩的变形和工程设计有一定的参考价值。     

Behaviour of Brittle Material in Multiple Loading Rates Under Uniaxial Compression

It is of great importance to investigate the effect of loading rate on the behaviour of brittle material such as concrete and rock because engineering structures are subjected to multiple loading conditions. Although material behaviour under single loading mode has been extensively studied, very limited research has been conducted to investigate the performance of brittle materials subjected to varying loading conditions. This paper presents an experimental study of the effects of single and multiple strain rates (ε) on cement mortar samples. The first set of samples was loaded at constant strain rates until failure. For the remaining samples, the first strain rate (0.005 mm/s) was applied to the sample up to a predetermined load, and then the second strain was initiated immediately by using the specially-designed gear system in place in the compression rig. As expected, the increase in strain rate showed an increase in peak strength of the sample with reduced ultimate strain. For multiple strain modes, it was observed that the highest peak strength occurred when the second strain was applied at 50 % of the peak strength of the first strain.     

Experimental investigations on the elastic parameters and uniaxial compressive strength of slate under freeze–thaw cycles

ABSTRACT

In this study, uniaxial compression experiments with seven different bedding angles and six numbers of freeze–thaw cycles were conducted to investigate the influences of freeze–thaw cycles on the elastic parameters and the uniaxial compressive strength of slate. The laws of the elastic parameters, uniaxial compressive strength and failure characteristics were analysed, and a new uniaxial compressive strength prediction model that considers the bedding angle and the number of freeze–thaw cycles as control variables was established and verified using the experimental data. The results showed that the uniaxial compressive strength, elastic modulus and shear modulus decreased exponentially with an increasing number of freeze–thaw cycles. However, the Poisson’s ratio increased linearly with an increasing number of freeze–thaw cycles. The uniaxial compressive strength initially decreased and then increased with increasing bedding angle. There are three forms of failure occurred during the tests: when the bedding angle was 0°≤β ≤ 26.6°, the splitting failure and shear failure occurred at the same time; when the bedding angle was 26.6°≤β ≤ 83.0°, sliding failure occurred along the bedding plane; and when the bedding angle was 83.0°≤β ≤ 90°, splitting failure occurred along the axial direction of sample.     

攀枝花尖山矿区细中粒辉长岩力学特性试验研究

对攀枝花钒钛磁铁矿尖山矿区的细粒和中粒辉长岩进行了单轴压缩、常规三轴压缩、抗拉强度和软化等系列岩石力学试验,研究了岩石结构(矿物颗粒大小)、水和围压等因素对岩石强度和变形特性的影响。结果表明,细粒辉长岩单轴抗压强度、弹性模量和压拉比均高于中粒辉长岩,但在三轴压缩情况下,两种岩石的峰值强度、残余强度和弹性模量差异较小;与中粒辉长岩相比,细粒辉长岩的峰值强度的黏聚力C较大,而峰值强度的内摩擦角φ较小;随着围压的增长,辉长岩峰值强度、残余强度与围压近似呈线性关系,剪切破坏角减小,平均模量E增长不明显,割线模量E50增长较显著;辉长岩的软化系数较高,在水的作用下弹性模量降低,泊松比升高。     

高温状态下加载速率对石灰岩力学效应研究

采用美国MTS810电液伺服材料试验机,对200 ℃高温状态下的石灰岩试件进行了5级不同加载速率下的单轴压缩试验,考察了石灰岩的全应力-应变曲线,给出了其峰值强度、峰值应变、弹性模量E随加载速率的变化特征。研究结果表明,(1) 石灰岩的峰值强度、弹性模量在加载速率v = 0.000 5～0.005 mm/s内,随加载速率增大呈急剧上升现象;当v = 0.005 0～0.5 mm/s内,随加载速率增大呈下降趋势;当v >0.5 mm/s后,呈现上升趋势。(2) 石灰岩的峰值应变在v = 0.000 5～0.005 mm/s内,随加载速率增大呈下降趋势;在v = 0.005～5.0 mm/s加载速率区段,峰值应变呈现缓慢上升现象。研究结果可为相关岩体的爆破效应及评价岩石工程的稳定性提供参考。     

Anisotropic mechanical properties of zircon and the effect of radiation damage

This study provides new insights into the relationship between radiation-dose-dependent structural damage due to natural U and Th impurities and the anisotropic mechanical properties (Poisson’s ratio, elastic modulus and hardness) of zircon. Natural zircon samples from Sri Lanka (see Muarakami et al. in Am Mineral 76:1510–1532, 1991) and synthetic samples, covering a dose range of zero up to 6.8 × 10¹⁸ α-decays/g, have been studied by nanoindentation. Measurements along the [100] crystallographic direction and calculations, based on elastic stiffness constants determined by Özkan (J Appl Phys 47:4772–4779, 1976), revealed a general radiation-induced decrease in stiffness (~54 %) and hardness (~48 %) and an increase in the Poisson’s ratio (~54 %) with increasing dose. Additional indentations on selected samples along the [001] allowed one to follow the amorphization process to the point that the mechanical properties are isotropic. This work shows that the radiation-dose-dependent changes of the mechanical properties of zircon can be directly correlated with the amorphous fraction as determined by previous investigations with local and global probes (Ríos et al. in J Phys Condens Matter 12:2401–2412, 2000a; Farnan and Salje in J Appl Phys 89:2084–2090, 2001; Zhang and Salje in J Phys Condens Matter 13:3057–3071, 2001). The excellent agreement, revealed by the different methods, indicates a large influence of structural and even local phenomena on the macroscopic mechanical properties. Therefore, this study indicates the importance of acquiring better knowledge about the mechanical long-term stability of radiation-damaged materials.